Usually, one wants to make bumper beams as stiff as possible so that their collapsing is delayed when they hit a barrier. In particular for a rear bumper, one of the most common accidents is however a vehicle backing onto a narrow pole on a parking lot. A bumper beam that, in a backing accident, is strong when hitting a barrier may not yield when hitting a narrow pole close to the centre of the vehicle. It will instead fold and collapse completely. WO 00/66400 shows a bumper beam in the form of a hat beam that has its web height reduced towards its centre.